@farmerman,
Salt.
Here's the recipe and procedure from David Lebovitz's blog:
Always use organic or unsprayed lemons, since you're going to be eating the exterior (and I don't want to lose any of my readers prematurely.) I generally do 8-10 lemons at a time, but be sure to buy a few extra lemons for juicing a couple of days later, in case you need to add additional liquid to keep the lemons in the jar submerged while they 'do their thing'. And never use ordinary table salt, which has a harsh, chemical taste. (Check the ingredients to make sure your salt is pure.) As always, I recommend sea salt or kosher salt.
Moroccan Preserved Lemons
* Scrub the lemons with a vegetable brush and dry them off.
* Cut off the little rounded bit at the stem end if there's a hard little piece of the stem attached. From the other end of the lemon, make a large cut by slicing lengthwise downward, stopping about 1-inch (3 cm) from the bottom, then making another downward slice, so you've incised the lemon with an X shape.
* Pack coarse salt into the lemon where you made the incisions. Don't be skimpy with the salt: use about 1 tablespoon per lemon.
* Put the salt-filled lemons in a clean, large glass jar with a tight-fitting lid. Add a few coriander seeds, a bay leaf, a dried chili, and a cinnamon stick if you want. (Or a combination of any of them.)
* Press the lemons very firmly in the jar to get the juices flowing. Cover and let stand overnight.
* The next day do the same, pressing the lemons down, encouraging them to release more juice as they start to soften. Repeat for a 2-3 days until the lemons are completely covered with liquid. If your lemons aren't too juicy, add more freshly-squeezed lemon juice until their submerged, as I generally have to do.
* After one month, when the preserved lemons are soft, they're ready to use. Store the lemons in the refrigerator, where they'll keep for at least 6 months. Rinse before using to remove excess salt.
To use: Remove lemons from the liquid and rinse. Split in half and scrape out the pulp. Slice the lemon peels into thin strips or cut into small dices. You may wish to press the pulp through a sieve to obtain the flavorful juice, which can be used for flavoring as well, then discard the innards.
and the one at Simply Recipes:
http://simplyrecipes.com/recipes/how_to_make_preserved_lemons/
How to Make Preserved Lemons
Ingredients
8-10 Meyer lemons*, scrubbed very clean
1/2 cup kosher salt, more if needed
Extra fresh squeezed lemon juice, if needed
Sterilized quart canning jar
* You don't need to use Meyer lemons, regular lemons will do, it's just that the milder Meyer lemons work very well for preserving in this way.
Method
1 Place 2 Tbsp of salt in the bottom of a sterilized jar.
One by one, prepare the lemons in the following way. Cut off any protruding stems from the lemons, and cut 1/4 inch off the tip of each lemon. Cut the lemons as if you were going to cut them in half lengthwise, starting from the tip, but do not cut all the way. Keep the lemon attached at the base. Make another cut in a similar manner, so now the lemon is quartered, but again, attached at the base.
3 Pry the lemons open and generously sprinkle salt all over the insides and outsides of the lemons.
4 Pack the lemons in the jar, squishing them down so that juice is extracted and the lemon juice rises to the top of the jar. Fill up the jar with lemons, make sure the top is covered with lemon juice. Add more fresh squeezed lemon juice if necessary. Top with a couple tablespoons of salt.
5 Seal the jar and let sit at room temperature for a couple days. Turn the jar upside down occasionally. Put in refrigerator and let sit, again turning upside down occasionally, for at least 3 weeks, until lemon rinds soften.
6 To use, remove a lemon from the jar and rinse thoroughly in water to remove salt. Discard seeds before using. Discard the pulp before using, if desired.
7 Store in refrigerator for up to 6 months.
Note:
You can add spices to the lemons for preserving - cloves, coriander seeds, peppercorns, cinnamon stick, bay leaf.
